The present invention relates to a method for providing a roll for a paper machine with sliding bearings, the roll comprising a stationary roll axle and a tubular roll mantle arranged to revolve around it, to which roll is applied a radial external load in at least one direction, in which method the roll mantle is supported on the roll axle by means of hydraulic sliding bearing members which are arranged to accomplish radial fitting with bearings of the roll mantle in the loading direction and in the lateral direction with respect to it, and the axial fitting with bearings of the roll mantle, and which sliding bearing members are loaded by means of a hydraulic medium, the feed of which to the sliding bearing members is controlled by means of control valves.
The Applicant""s earlier U.S. Pat. No. 98,320 discloses a method for providing the roll mantle of a tubular roll intended for a paper machine or equivalent with sliding bearings, and a tubular roll that makes use of the method. In the solution according to the patent in question, the fitting with bearings corrects the position of the roll mantle in the perpendicular direction to the nip when it differs from the preset value. The said solution does not correct the position in the direction of the nip, but allows the mantle to be positioned in accordance with the position of the backing roll. In this solution, the roll mantle of the tubular roll is allowed to shift over a considerable distance in the main loading direction, giving as an example of the permitted movement of the mantle approximately 25 mm in both directions. In U.S. Pat. No. 98,320, the question is of so-called fitting with sliding bearings with stroke, which functions as such in many applications but, for example, in belt presses and in the calenders (Optiload(trademark) calender) described in the Applicant""s FI-patent no. 96334, this type of fitting with sliding bearings with stroke is not suitable. An important aim of the present invention is, therefore, to provide a method by means of which the fitting with sliding bearings of the roll mantle arranged to revolve around the stationary axle can be arranged in such a way that the mantle will remain in place with respect to the axle, allowing relatively high loads in the direction of the nip. Another aim of the invention is to provide a roll which is fitted with sliding bearings in such a way that the roll mantle will remain in place with respect to the axle, allowing relatively high loads in the direction of the nip. The roil is preferably a variable crown roll.
A further aim of the invention is to accomplish control of pressures in the sliding bearing members in such a way that, in the members, there will be a pressure corresponding to each loading situation respectively, so that the mantle is xe2x80x9cexpandedxe2x80x9d as little as possible. In other words, the pressure opposite to loading should be allowed to increase, while the pressure parallel to the direction of loading should be as low as possible. A certain amount of pressure should, however, always be applied to the members so that the shoe will maintain light contact with the mantle and ensure sufficient lubrication. If the change caused by the movement of the control valve stem in the surface area of the feed opening of the hydraulic medium in the control valve is constant, the pressures will behave as shown in FIG. 1. In FIG. 1, reference mark A refers to the pressure prevailing in the sliding bearing member effective in the direction opposite to loading, reference mark B to the pressure prevailing in the sliding bearing member effective in the direction of loading, FL refers to the force generated by the bearings and FE to the force expanding the mantle. In accordance with FIG. 1, the force FE expanding the mantle is strong around the centre position of the stem. Should the expansion of the mantle not have to be considered, it would be advisable to realise the bearing in the manner shown in FIG. 1. The force amplification in the control system is greatest when the pressure changes under both bearing members, and not just under one bearing member.
To achieve the above aims, it is characteristic of the method relating to the invention that when the load is applied to a sliding bearing member effective in the loading direction, the loading force applied to the sliding bearing member subject to an external load due to the loading is compensated for automatically, essentially immediately, without an external control instruction, by means of the movement of the control valve stem of the sliding bearing member subject to loading, the said control valve bringing about a rapid increase in the pressure of the hydraulic medium in the sliding bearing member subject to the external load, at the same time as the pressure of the medium supplied to the sliding bearing member located radially on the opposite side of the roll axle in the loading direction is adjusted to a certain minimum value, whereby the bearings of the roll mantle are rendered so as to be essentially without stroke.
The roll relating to the invention is, on the other hand, characterised in that the arrangement comprises a control valve which compensates for the loading force applied to the bearing member located in the loading direction by an external load automatically, essentially immediately, controlled by the movement of the bearing member in question brought about by the loading, by bringing about a rapid increase in the pressure of the hydraulic medium in the sliding bearing member subject to the external load, at the same time as the pressure of the medium supplied to the sliding bearing member located radially on the opposite side of the roll axle in the loading direction is adjusted to a certain minimum value, and that the bearings of the roll mantle have been rendered so as to be essentially without stroke.
As concerns the control of pressures, the situation shown in FIG. 2 can be achieved by means of the solution relating to the invention, in which the force FE expanding the mantle can be maintained low, while obtaining a high bearing force FL. The bearing force only needs to be adjusted by means of one bearing member (the bearing member opposite to the load) to prevent the deformation of the mantle.